Describe the bug
Running make check yields two test failures: gdimagegrayscale/basic and gdimagecopyresampled/bug00201

To Reproduce
Steps to reproduce the behavior:

1. Download the libgd-2.3.3 tarball
2. export CXXFLAGS="-O3 -march=skylake"
3. export CFLAGS="-O3 -march=skylake"
4. ./configure --disable-rpath --disable-shared --with-png --with-jpeg --enable-gd-formats --with-tiff --with-freetype --with-fontconfig --with-liq --with-xpm
5. make
6. make check

Expected behavior
All test run by make check pass.

Actual results

====================================
   GD 2.3.3: tests/test-suite.log
====================================

# TOTAL: 208
# PASS:  206
# SKIP:  0
# XFAIL: 0
# FAIL:  2
# XPASS: 0
# ERROR: 0

.. contents:: :depth: 2

FAIL: gdimagecopyresampled/bug00201
===================================

gdimagecopyresampled/bug00201.c:73: Total pixels changed: 268 with a maximum channel difference of 131.
FAIL gdimagecopyresampled/bug00201 (exit status: 1)

FAIL: gdimagegrayscale/basic
============================

gdimagegrayscale/basic.c:25: Total pixels changed: 16800 with a maximum channel difference of 131.
FAIL gdimagegrayscale/basic (exit status: 1)

Environment (please complete the following information):

• OS: Ubuntu 20.04
• Version : GD 2.3.3
• libpng 1.6.37-2
• libjpeg8 8c-2ubuntu8
• libjpeg-turbo8 2.0.3-0ubuntu1.20.04.3
• libtiff5 4.1.0+git191117-2ubuntu0.20.04.14
• libfreetype6 2.10.1-2ubuntu0.4
• libfontconfig1 2.13.1-2ubuntu3
• libimagequant0 2.12.2-1.1
• libxpm4 1:3.5.12-1ubuntu0.20.04.2